TOKYO, July 5 (Reuters) - Japan's Government Investment Pension Fund (GPIF) reported on Friday an investment gain of 21.4 trillion yen ($133.3 billion) in January-March as equities rallied strongly in Japan and globally.

The fund gained 9.52% for the quarter, raising its total assets to 246 trillion yen ($1.53 trillion), it said in its 2023 annual report.

It is closely watched by global financial markets due to its colossal size.

The fund posted an annual gain of 45.4 trillion yen ($282.5 billion) in the 12 months to the end of March 2024, boosted by a stronger than expected U.S. economy and increasing expectations of lower interest rates in major western economies, the report said.

The weakening yen also bolstered the value in yen of its international holdings, GPIF President Masataka Miyazono told a press briefing.

The fund's Japanese stock portfolio gained 18.2% in the Jan-March quarter while its foreign stock portfolio grew 15.8%.

Over the same period, Japan's Nikkei stock average gained 20.6% and the S&P 500 gained 10.2%.

The fund's foreign bond portfolio grew 5.4% but its Japanese bond portfolio fell 0.6%. ($1 = 160.7200 yen) (Reporting by Anton Bridge; Editing by Chang-Ran Kim and Kim Coghill)